New Sewer Connection and De Watering Lytham

We were asked to carry out a new sewer connection for a recently built house by a client. We initially attended site to check depths and size of the public sewer and get any information we required to price for the job. A quote was e mailed with all the details of the proposed works and accepted by the customer.

With a new sewer connection you have to fill in an application pack for united utilities as you are connecting onto their sewer. The owner of the new drain to be connected to the sewer fills out part 1 of the application and then we completed part 2 which included method statements, qualifications, risk assessments and all our company / insurance details for united utilities to firstly approve our company to carry out the works and secondly to approve the method in which we were carrying out the works.

Once approval was granted by united utilities a meeting with the council was arranged to see what their requirements where regarding traffic management and any specific requirements regarding the highway. As a result of the meeting we found out a full road closure was required and because of the classification of the road the tarmac was to be replaced the full way across the road at the width of the excavation. The application was then made to the council with the proposed start dates so they could advertise the closure.

The next step of the job was to carry out a trial hole which is an excavation to find out at what depth we would find the water level; this was carried out as although we are aware of the bad ground conditions in Lytham St Annes and expected de watering to be required, it is always worth carrying out the trial to potentially save the customer costs.

De Watering is required to lower the water table within the excavation so you can attain the depth required to carry out your works, The de watering was done using WellPoint’s which are small-diameter (about 50 mm) tubes with slots near the bottom that are inserted into the ground from which water is drawn by a vacuum generated by a dewatering pump. WellPoint’s are typically installed at close centers in a line along or around the edge of an excavation. As a vacuum is limited to 0 bar, the height to which water can be drawn is limited to about 6 meters (in practice).

Following the trial hole a CCTV survey was carried out using a Crawler CCTV unit to check the condition of the public sewer which we would be connecting to. We also used the information to locate existing laterals and pin point where we would be making our connection.

On day 1 of the works the first job was to erect traffic management for traffic diversions and close the road off so we could start unloading the equipment and begin the excavation. A site survey was carried out to identify any existing faults surrounding the excavation area and photographic evidence was obtained. The tarmac was then cut and removed and the excavation began. We excavated by hand to find all the services in the road which included, electric, gas main and water, once this was carried out an excavator was used to dig down towards the sewer. Once we excavated to approximately 1.2m we installed trench sheet supports to shore up and avoid any collapse of the trench the excavation then continued until we reached the water table.

The next step was for the De watering to be installed, this involved 4 bore holes being sunk into the trench and pipes where inserted and connected to a large vacuum pump. Because the ground conditions in the area where mainly sand we also installed a large silt trap / filter to catch any sand debris and avoid it going back into the main sewer. The pump was then started and de watering began. The water level steadily started receding and we began excavating towards the sewer. At this point we noticed that the water level had started to rise again due to the tide reaching its peak, once the tide turned the de watering equipment again started to draw the water down to the required level.
Once we managed to get the water down to the required depth we cored a hole into the public sewer ready to make our saddle connection. The pipe was then laid back to the property and connected up.

The excavation at the deepest point next to the sewer was backfilled until we were above the original water table and the dewatering was switched off and removed. The next stage was to remove the trench sheets and the back filling and compacting began. The backfill had to be compacted in stages as this is the correct way to construct a road, this was done up until the point we were ready to lay tarmac.

At this point the entire area was washed down with a high pressure jetting vehicle to clean any remaining debris around the original excavation.

The Tarmac was then laid to council specifications and the road was re-opened.

Should you have a similar problem or require some advice on new sewer connections do not hesitate to contact us on 01253 811181 or pipescan@btconnect.com

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